The pump can be connected hydraulically by way of the valve to a low-pressure fuel feed unit. This is connected hydraulically on the output side to the nozzle unit. The start of injection and injection quantity are determined by the valve and its valve actuating device. The compact construction of the pump/nozzle device results in a very small high-pressure volume and a great hydraulic rigidity. Extremely high injection pressures of approximately 2000 bar are thus made possible. This high injection pressure in conjunction with good controllability of the start of injection and the injection quantity make possible a significant reduction in emissions whilst simultaneously keeping fuel consumption low when using the internal combustion engine.
A pump/nozzle device is known from DE 198 35 494 C2 having a pump and a valve with a valve element which controls the hydraulic connection of an auxiliary control chamber with an outflow duct. The outflow duct is connected hydraulically to the pump and to a nozzle unit. An inflow duct is provided which is connected hydraulically to the auxiliary control chamber. A piezoelectric valve actuating device, which allows the valve element to be adjusted between two end positions, is associated with the valve element. In a first end position of the valve element the outflow duct is connected hydraulically to an auxiliary control chamber and the latter in turn to the inflow duct. In a second end position of the valve element the outflow duct is disconnected hydraulically from the auxiliary control chamber and the valve element is in a valve seat of the valve.
In the first end position of the valve element, during a delivery stroke of the pump, fluid is drawn in by the pump from the inflow duct by way of the auxiliary control chamber and the outflow duct. During a working stroke of a pump piston of the pump, in the first end position of the valve element fluid is forced back by the pump by way of the inflow duct and the auxiliary control chamber into the outflow duct. In the second end position of the valve element, during the delivery stroke of the pump piston no fluid can be forced back on account of the absence of a hydraulic connection between the outflow duct and the auxiliary control chamber and the outflow duct, and the pump piston generates high pressure. When a predetermined pressure threshold is exceeded, a nozzle needle in the nozzle unit opens a nozzle in the nozzle unit and an injection of the fluid takes place. The end of injection is determined by the fact that the valve element is moved into its first end position by means of the actuator, thereby allowing fluid to flow back by way of the outflow duct into the auxiliary control chamber and the inflow duct, as a result of which the pressure in the pump and thus also in the nozzle unit decreases, which in turn leads to closure of the nozzle unit.
Precise metering of fuel through the pump/nozzle device presupposes a capability to control the valve in an extremely precise manner.
A method for controlling an injection valve with a piezoelectric actuator is known from EP 1179129 B1, in which during opening and closing of the valve the piezoelectric actuator is initially recharged with a first boosting charge with a maximum gradient and thus executes a partial stroke. After a recharging interval of a predetermined time period, the piezoelectric actuator is then charged in the same direction with a second partial charge on the final stroke, whereby the gradient for the second partial charge is less than the maximum gradient of the first partial stroke.